Thermoplastic materials which are used for molding are handled easier if provided in the form of free-flowing granules or pellets of uniform size. The methods of forming such pellets include devices which extrude the plastic material in the form of continuous rods which are severed soon after extrusion and after being cooled in a water bath or while under water at the face of the die.
In the latter case hot thermoplastic material is extruded through extrusion orifices in a die plate. The extruded material pass through the orifices in the form of hot thermoplastic rods from a flat portion of the die plate into a water filled chamber having water circulating therein. While immersed in the water, the rods are cut into pellets by knives mounted on a rotor driven by a shaft. One such device is a Farrel underwater pelletizer manufactured by Farrel Corp. which has been in use for a number of years.
The devices as previously used have some drawbacks. All the knife blades previously used allow the circulating water to remain in contact with the die face. The water can cause uneven cooling of the die face which causes plugging of the orifices. Thus slower extrusion rates occur in the coolest area of the die face with the slower extruded material being cut into shorter lengths and the faster extruded material cut in longer lengths. The uneven length produces "fines" which may cause difficulty in some final applications. Additionally, the cooler pellets may not be cleanly cut and retain a "smear" of uncut material referred to as "tails". These tails may become separated during handling causing additional "fines".